The present invention relates to a spectrophotometer and particularly to a double-beam spectrophotometer.
In general, since the photoelectron multiplying function is just suitable for designing of a doublebeam type spectrophotometer, a photomultiplier is used as a detector in the spectrophotometer to conduct waveform scanning (for example U.S. Pat. No. 4,455,097).
An electrical block diagram of the spectrophotometer of the prior art utilizing the photomultiplier of this type is shown in FIG. 2.
In this spectrophotometer, the light from the light source 1 is once separated depending on the wavelength into various light beams, these are then chopped into the light beam passing through reference cell 3 and the light beam passing through a sample cell 4, such light beams are received by a photomultiplier 5. The optical current from photomultiplier 5 is amplified by a preamplifier 6, divided into the reference and sample currents by switches 7, 8. The reference signal is amplified by an amplifier 9 and is then fed back to the photomultiplier 5 through a negative voltage circuit 11, meanwhile the sample signal is amplified by an amplifier 10 and is then input to an A/D converter.
In this spectrophotometer, control is carried out so that the reference signal is fed back to a dinode of the photomultiplier 5 via the amplifier 9 and negative voltage circuit 11 to make constant the reference signal. The sample signal directly becomes a transmittance of sample and is subject to the A/D conversion under the condition that the reference signal is kept to a constant value and is then output as the data.
In the spectrophotometer of the prior art, a photomultiplier is used for a detector and it is certainly advantageous to use the photomultiplier for performing the photoelectron multiplying function. On the contrary, there is a disavantage in that the photomultiplier is very expensive and results in a rise of cost, of manufacture, further the electrical circuit is also complicated because it requires a negative voltage circuit and thereby the spectrophotometer as a whole becomes expensive.